The same thing that happens when any road vehicle meets a break down they didn't see in time, except the massive truck is more likely to win.

http://divine.vic.gov.au/lifestyle/health-and-wellbeing/road... has an account of a collision with a road train:

"The road train driver had slowed down to make a left-hand turn at a crossroads and Scotty had slammed into him at over 100 kilometres an hour. According to the autopsy, Scotty was over the legal alcohol limit. The driver of the road train kept going once he had completed the corner. I was later told he thought he felt a shift in the load as he slowed the vehicle to negotiate the corner."

Anyone wanting to make autonomous trucks perfect should remember the bar is actually pretty low...